A method for detecting short circuits is described in a book entitled "Relaisschutztechnik in Elektroenergiesystemen"(Protection in Electrical Power Systems), Dr.-Ing. Heinz Clemenz, Dr.-Ing. Klaus Rothe, VEB Verlag Technik, Berlin, 1980, pages 64 through 66. In this conventional method, the absolute impedance value formed from current and voltage during a short-circuit is checked to see whether it lies within a circle with a center point defined by the start of the monitored section of a power supply line. The circle radius is determined by the size of the monitored section. Although this method may generate the tripping signal relatively quickly, it has a disadvantage in that the circular tripping characteristic adapts poorly to the conditions of an electrical power supply line; if an unfavorable value is selected for the circle radius, for example a value that encompasses all possible short-circuit faults on the monitored line section, the circular tripping characteristic may also include impedance values that cannot be attributed to a short-circuit fault but which occur under certain load conditions. On the other hand, selecting a circle radius so small that the circle cannot produce false tripping means that some of the short-circuit faults may not be detected. The conventional method therefore achieves a relatively high tripping speed, but is able to detect only some of the short-circuit faults that can occur.
A further impedance protection method is described in the operating instructions of the PD 551 impedance protection device manufactured by AEG, pages III/29 ff. This method uses a polygonal tripping characteristic. The resistance and reactance components of these impedance values are checked to see whether they lie within this polygon. The method requires a relatively large amount of time in order to form a tripping signal, since the data window needed for calculating the resistance and reactance components must be relatively large. On the other hand, the polygonal method detects a larger number of the possible short-circuit faults than does the method using the circle, since the polygonal characteristic can be better adapted to the conditions of the power supply line.